Expression of inositol 1,4,5-trisphosphate receptors changes the Ca²⁺ signal of Xenopus oocytes

The receptors for the second messenger inositol 1,4,5-trisphosphate [Ins( 1,4,5)P₃] form a family of closely related proteins that play an important role in regulating the free intracellular Ca²⁺ concentration. To test the hypothesis that changing the expression level of Ins(1,4,5)P₃ receptors could alter the Ins(1,4,5)P₃-mediated Ca²⁺ signal, we overexpressed Ins(1,4,5)P₃ receptor type 1 (InsP₃R-1) or type 3 (InsP₃R-3) in Xenopus laevis oocytes. Expression of InsP₃R-1 increased the velocity of the propagating waves of intracellular Ca²⁺ release but did not affect the Ins(1,4,5)P₃-induced entry of extracellular Ca²⁺ across the plasma membrane. In contrast, expression of InsP₃R-3 did not affect Ins(1,4,5)P₃-induced release of intracellular Ca²⁺ but markedly increased the magnitude and duration of Ca²⁺ influx. Immunolocalization studies revealed InsP₃R-3 at the endoplasmic reticulum, with a relatively stronger signal at or near the plasma membrane. The results suggest that changing the expression level of an InsP₃R can alter the Ins(1,4,5)P₃-mediated Ca²⁺ signal and that InsP₃R-1 and InsP₃R-3 may have different biological functions.